The invention is used for the determination of gas concentration of a gas in chamber, such as a vacuum-coating chamber for a PVD (plasma vapor deposition) method with an arrangement of cathodes in the chamber, a gas inlet into the chamber, a separate measurement chamber for accommodating a measuring device, which preferably is a measuring head of a mass spectrometer, a connecting line between the two chambers with a pressure stage, as well as a pump stand for producing a pressure gradient between the interior of the vacuum chamber and the interior of the measurement chamber.
A method and an apparatus are known for the reactive deposition of a metal compound on a substrate (European patent 0 121 019). For this method, the amount of reactive gas in the chamber is determined by withdrawing the excess reactive gas in the chamber at a place, which is spatially removed from the reaction zone. This publication furthermore describes an apparatus, which contains a mass spectrometer, which generates a control command. An adjustable control circuit is connected to the mass spectrometer in order to generate a specific peak signal from the different signals initially generated by the mass spectrometer.
An apparatus for determining the concentration of a gas in a vacuum chamber is well known in the art. By means of a pressure stage, this apparatus disengages a portion of the gas atmosphere of a vacuum chamber at a pressure of between, for example, 1.times.10.sup.-3 mbar and 5.times.10.sup.-2 mbar and generates before the measuring head of a mass spectrometer a pressure preferably of between 1.times.10.sup.-5 mbar and 5.times.10.sup.-4 mbar. The pressure stage consists, for example, of an aperture, which is pumped differentially with a separate pumping means.
The reactive gas concentration, which has a critical effect on the coating result of the vacuum deposition process, for example, in the case of a PVD (plasma vapor deposition) method, is that located immediately in front of, or in contact with the target surface. On the other hand, previously known state of the art methods consider only a composition of the gas atmosphere which exists at some distance from the cathode. Due to an inhomogeneous gas distribution at or near the surfaces at which the reaction takes place, the chemical composition at some distance from the cathode can differ substantially from the chemical composition of interest. Furthermore, because of the characteristics of the measuring equipment, a higher proportion of the reactive gas in the atmosphere to be measured would likewise improve the signal-to-noise ratio of the signal at the mass spectrometer.
For these reasons, an atmosphere, which is as close as possible to the cathode surface, should be made available to the mass spectrometer for analysis.